Energy & Biomass Pyramids: Videos & Practice Problems

Energy and biomass pyramids illustrate ecosystem structure and trophic efficiency. Energy pyramids, always upright, show energy transfer between trophic levels over time, with low trophic efficiency averaging around 10%. In contrast, biomass pyramids capture a snapshot in time and can take various shapes, including inverted. Factors contributing to low trophic efficiency include incomplete ingestion, incomplete digestion, metabolic energy use, and heat loss. Additionally, biomagnification highlights how pollutants concentrate at higher trophic levels, impacting top predators significantly.

Energy & Biomass Pyramids

Energy and biomass pyramids serve as essential visual tools for understanding ecosystem structure and trophic efficiency. Energy pyramids illustrate how energy is transferred between trophic levels over time, highlighting that only a small fraction of energy from one level is passed to the next. This transfer is typically low, with an average trophic efficiency of around 10%. The units used in energy pyramids are expressed as joules per meter squared per month, emphasizing the energy available per area over a specific time period. Due to this energy transfer pattern, energy pyramids maintain an upright shape.

In contrast, biomass pyramids focus on the biomass present at a single moment in time, which allows for various shapes, including upright, inverted, or diamond configurations. For instance, an inverted biomass pyramid may occur when the biomass of primary consumers, such as zooplankton, exceeds that of primary producers, like phytoplankton. In this scenario, 8 grams of phytoplankton can support 42 grams of zooplankton, illustrating a high consumption rate. However, the rapid reproduction of phytoplankton compensates for their consumption, which is not captured in a snapshot of biomass. The units for biomass pyramids are expressed in grams per meter squared, reflecting mass per area without a time component.

The low trophic efficiencies can be attributed to several factors: incomplete ingestion, where not all available energy is consumed; incomplete digestion, where some ingested energy is lost as waste; energy used for metabolism and cellular respiration; and heat loss during energy transfers. Each of these factors contributes to the limited energy available for transfer to higher trophic levels.

Another critical concept is biomagnification, which describes how pollutants become increasingly concentrated as they ascend the food chain. At lower trophic levels, pollutant concentrations are relatively low, but as these pollutants are not easily digested or excreted, they accumulate in the biomass. Consequently, organisms at the top of the food chain experience the most significant effects of biomagnification, facing higher concentrations of harmful substances.

Understanding these concepts is vital for analyzing ecological interactions and the implications of energy flow and pollutant dynamics within ecosystems.

Energy & Biomass Pyramids Example 1

Understanding energy efficiency in dietary choices is crucial for evaluating the sustainability of food systems. When considering why a plant-based diet is more energy efficient for humans compared to a meat-based diet, it is essential to refer to the concept of energy pyramids. An energy pyramid illustrates the distribution of energy across different trophic levels in an ecosystem, with primary producers, such as plants, at the base, containing the highest amount of energy stored in their biomass.

As one moves up the pyramid to primary, secondary, and tertiary consumers, the energy available decreases significantly. This decline occurs because energy transfers between trophic levels are inefficient; only about 10% of the energy from one level is passed on to the next. Consequently, if humans consume meat, which requires multiple energy transfers (from plants to animals and then to humans), they are not maximizing energy efficiency. In contrast, by consuming plants directly as primary consumers, humans minimize these energy transfers, thereby maximizing the energy they receive.

For instance, while it may be true that humans can obtain more energy from a mass of plants than from meat in some cases, this is not universally applicable, especially when considering the fat content in meat. The key factor influencing energy efficiency is not merely the energy content of food but rather the number of energy transfers involved in obtaining that energy.

Additionally, while animal-based products tend to be more expensive due to the greater resources required for their production, this does not directly address the energy efficiency aspect. The most accurate statement regarding energy efficiency is that a given amount of solar energy can produce significantly more plant biomass than animal biomass. This principle highlights the inefficiencies inherent in energy transfer through the food chain, where energy is lost at each step. Therefore, adopting a plant-based diet is a more energy-efficient choice for humans, aligning with ecological sustainability.

True or false: Most of an ecosystem’s net primary productivity is ultimately used for the growth of primary consumers within the ecosystem.

• A. True because this is the only food source available to primary consumers.
• B. True because primary consumers are the most efficient organisms at converting their food into biomass.
• C. False because primary consumers consume most of the plant matter, but don’t convert most of it to biomass.
• D. False because primary consumers don’t consume most of the plant matter, & they do not convert most of the food they ingest into biomass.

Answer: (D)

Which of the following is a reason that such a small fraction of the energy that reaches the Earth as solar radiation ends up at the top of the food chain?

• A. Plants only absorb a very small fraction of the total solar energy that reaches Earth.
• B. Lots of energy is dissipated as heat at all trophic levels.
• C. Incomplete ingestion – consumers do not eat all the food available to them.
• D. All of the above.

Answer: (D)

Due to biomagnification, there are certain species with a high concentration of heavy metals (e.g., mercury), leading to dietary advisories for vulnerable demographics (e.g., pregnant women and young children) to avoid these species. Given that these advisories are often most critical for species at the top of long marine food chains, which species would this advice most likely apply to?

• A. Cows (primary consumers, primarily eat grass).
• B. Shrimp (secondary consumers, feed on zooplankton).
• C. Alligators (secondary/tertiary consumers, feed on many species).
• D. Tuna (top predators, feed on predatory fish & invertebrates).

Answer: (D)

Here’s what students ask on this topic:

What is the difference between energy pyramids and biomass pyramids?

Energy pyramids and biomass pyramids are both visual tools used to represent ecosystem structure, but they focus on different aspects. Energy pyramids illustrate the flow of energy through trophic levels over time, always maintaining an upright shape due to low trophic efficiency, which averages around 10%. This means only a small fraction of energy is transferred to the next level. The units are typically in energy per area per time, such as joules per meter squared per month. In contrast, biomass pyramids capture a snapshot of the biomass present at each trophic level at a specific moment in time. They can take various shapes, including upright, inverted, or diamond-shaped, depending on the ecosystem dynamics. The units are in mass per area, like grams per meter squared, without a time component.

Why are trophic efficiencies generally low in ecosystems?

Trophic efficiencies are generally low, averaging around 10%, due to several factors. First, incomplete ingestion occurs when not all available energy in one trophic level is consumed by the next. Second, incomplete digestion means that even consumed energy is not fully assimilated, with some lost as waste. Third, energy is used in metabolism and cellular respiration, reducing the amount available for transfer to the next level. Lastly, energy is lost as heat during each transfer. These factors collectively contribute to low trophic efficiency, meaning only a small fraction of energy is passed up the food chain, limiting the energy available to higher trophic levels.

What is biomagnification and how does it affect ecosystems?

Biomagnification is the process by which pollutants become more concentrated as they move up the food chain to higher trophic levels. Pollutants that cannot be digested or excreted accumulate in the biomass of organisms. As these pollutants are transferred up the food chain, their concentration increases, significantly impacting organisms at the top. This can lead to harmful effects on top consumers, including health issues and population declines. Biomagnification highlights the importance of understanding pollutant dynamics in ecosystems and the need for pollution control to protect biodiversity and ecosystem health.

How do energy pyramids maintain an upright shape?

Energy pyramids maintain an upright shape because they represent the flow of energy through trophic levels over time, with low trophic efficiency averaging around 10%. This means that only a small fraction of energy is transferred from one level to the next, resulting in a decrease in available energy as you move up the pyramid. The upright shape reflects this decrease, as each higher trophic level has less energy available than the one below it. The units used in energy pyramids, such as joules per meter squared per month, emphasize the time component and the continuous flow of energy through the ecosystem.

Can biomass pyramids be inverted, and if so, why?

Yes, biomass pyramids can be inverted. This occurs when the biomass of a lower trophic level is less than that of a higher one at a specific moment in time. For example, in aquatic ecosystems, a small biomass of phytoplankton can support a larger biomass of zooplankton due to the high reproduction rate of phytoplankton. This allows them to sustain their population despite being consumed rapidly. The snapshot nature of biomass pyramids, which captures a single moment in time, allows for such variations in shape, including inverted pyramids, reflecting the dynamic interactions within ecosystems.